Olefins, particularly light olefins, have been traditionally produced from petroleum feedstocks by either catalytic or steam cracking. Oxygenates, however, are becoming an alternative feedstock for making light olefins, particularly ethylene and propylene. Promising oxygenate feedstocks are alcohols, such as methanol and ethanol, dimethyl ether, methyl ethyl ether, diethyl ether, dimethyl carbonate, and methyl formate. Many of these oxygenates can be produced from a variety of sources including synthesis gas derived from natural gas; petroleum liquids; and carbonaceous materials, including coal. Because of the relatively low-cost of these sources, alcohol, alcohol derivatives, and other oxygenates have promise as an economical, non-petroleum source for light olefin production.
One way of producing olefins is by the catalytic conversion of methanol using a silicoaluminophosphate (SAPO) molecular sieve catalyst. For example, U.S. Pat. No. 4,499,327 to Kaiser, discloses making olefins from methanol using a variety of SAPO molecular sieve catalysts. The process can be carried out at a temperature between 300° C. and 500° C., a pressure between 0.1 atmosphere to 100 atmospheres, and a weight hourly space velocity (WHSV) of between 0.1 and 40 hr−1.
Inui (J. Chemical Society Chem. Commun. p. 205, 1990) has shown that the selectivity to ethylene can be increased when methanol is contacted with a nickel-substituted SAPO-34 rather than an unsubstituted SAPO-34. In this case, nickel substitution occurred into the SAPO-34 framework.
In contrast to the work of Kaiser and Inui, metal incorporation may also take place post-synthesis, that is, following the synthesis of the molecular sieve framework. For example, U.S. Pat. No. 5,962,762 to Sun et al. teaches a process for converting methanol to light olefins using a metal-incorporated SAPO catalyst. An aqueous metal solution, preferably a nickel or cobalt containing solution, is adsorbed onto the SAPO molecular sieve by allowing the solution to remain in contact with the SAPO overnight at ambient conditions. The treated molecular sieve is then separated from the solution and dried. U.S. Pat. Nos. 5,625,104 and 5,849,968 to Beck at al. teach a process of incorporating alkali earth and alkaline earth metals into a zeolitic catalyst by pretreating the zeolite with an organosilicon or poly-oxo silicon compound followed by the treatment of a metal solution. U.S. Pat. No. 4,692,424 to Le Van Mao teaches a process for the dry incorporation of manganese ions on the external reactive sites of ZSM catalysts by adding a minimum amount of an aqueous manganese solution to form a malleable paste and extruding the paste under pressure.
Post-synthesis metal incorporation of zeolite catalysts is used for other processes as well. U.S. Pat. No. 6,084,142 to Yao et al. teaches treating a zeolite catalyst with a zinc component in an aqueous solution followed by steam treatment for the conversion of hydrocarbons to lower olefins. There is no teaching of conversion of methanol to olefins.
Yamamoto et al. (Microporous and Mesoporous Materials 44–45, Organic Functionalization of Mesoporous Molecular Sieves with Grignard Reagents, p. 459–464, 2001) teach post-synthesis organic functionalization of MCM-41 in a two step procedure. MCM-41 is first modified by alcohols, which leads to the esterification of surface silanol groups (converting Si—OH to Si—OR) and then allowed to react with a Grignard reagent R′MgX which converts Si—OR to Si—R′. The two step procedure must be followed since Si—OH spoils Grignard reagent R′MgX to form Si—O—MgX and R′—H. There is no teaching of conversion of methanol to olefins.
PCT Application WO 97/26989 teaches adding a transition metal hydrogenation component in a non-aqueous solvent to a non-zeolitic molecular sieve after synthesis for hydrocracking and catalytic dewaxing. The hydrogenation component is in the form of a sulfide, halide, oxide, carboxylate, and the like. There is no teaching of conversion of methanol to olefins.
In spite of the prior efforts to modify molecular sieves, the need to modify the surface, in particular the interior surface, of small pore molecular sieves such as silicoaluminophosphates (SAPO) remains. Consequently, there is still a need to find an improved molecular sieve or molecular sieve catalyst that exhibits high ethylene and/or propylene selectivity in the conversion of methanol to light olefins.